Key moments related to the search for the Boston Marathon bombing suspects, based on reports from the Middlesex County district attorney, Massachusetts State Police, and Boston police.

– At 5:10 p.m. Thursday, investigators of the bombings release photographs and video of two suspects. They ask for the public's help in identifying the men.

– Around 10:20 p.m., shots are fired on the campus of the Massachusetts Institute of Technology in Cambridge, just outside Boston.

– At 10:30 p.m., an MIT campus police officer who was responding to a disturbance is found shot multiple times in his vehicle, apparently in a confrontation with the Boston Marathon bombing suspects. He is later pronounced dead.

– Shortly afterward, two armed men reportedly carjack a Mercedes SUV in Cambridge. A man who was in the vehicle is held for about a half hour and then released unharmed at a gas station on Memorial Drive in Cambridge.

– Police soon pursue the carjacked vehicle in Watertown, just west of Cambridge.

– Some kind of explosive devices are thrown from the vehicle in an apparent attempt to stop police. The carjackers and police exchange gunfire. A transit police officer is seriously injured. One suspect, later identified as Suspect No. 1 in the marathon bombings, is critically injured and later pronounced dead.

– Authorities launch a manhunt for the other suspect.

– Around 1 a.m. Friday, gunshots and explosions are heard in Watertown, just outside Boston. Dozens of police officers and FBI agents converge on a Watertown neighborhood. A helicopter circles overhead.

– Around 4:30 a.m., Massachusetts state and Boston police hold a short outdoor news briefing. They tell people living in that section of eastern Watertown to stay in their homes. They identify the carjackers as the same men suspected in the marathon bombings. Overnight, police also release a photograph of a man believed to be Suspect No. 2, apparently taken from store video earlier in the evening at a 7-Eleven convenience store in Cambridge. He is wearing a grey hoodie-style sweatshirt.